Hydrolyzing coffee material, particularly partially extracted coffee grounds, to obtain an increased solids yield is well known in the art. For example, U.S. Pat. No. 2,573,406 to Clough et al. discloses a process for producing a soluble coffee which involves atmospherically extracting about 20% of the weight of the coffee, hydrolyzing a portion of the grounds in a suspension of about 1% sulphuric acid at 100.degree. C. for about one hour, adjusting the pH of the hydrolysate, filtering the hydrolysate, combining the same with the atmospheric extract and drying the combined extract. In another, similar process described in U.S. Pat. No. 2,687,355 to Benner et al., phosphoric acid is used in place of sulphuric acid. In still another process, disclosed in U.S. Pat. No. 3,224,879 to DiNardo et al., either alkaline or acid hydrolysis is carried out directly in the extraction train on coffee grounds that have been at least atmospherically extracted. Hydrolysis directly in the extraction train eliminates the separate hydrolysis step of the prior art processes and provides for adsorption of the alkaline or acid catalyst in the mass of spent coffee grounds.
As to the Clough et al. and Benner et al. processes, the batch hydrolysis reactions at relatively low temperatures require about one hour to complete, limiting the practicality of said processes on a commercial scale. Moreover, both Clough et al. and Benner et al. essentially aim for whatever hydrolysate results from operating at 100.degree. C. for one hour. Neither disclosure describes a method for nor the desirability of manipulating the hydrolysis conditions so as to affect the composition of the resulting hydrolysate. A similar deficiency is noted with respect to the DiNardo disclosure.
It is also widely recognized in the art that cellulosic material containing predominantly carbohydrate polymers and lignins may be hydrolyzed with an acid catalyst under short time high temperature conditions. However, if the cellulosic material is not relatively pure the hydrolysis reaction will produce undesirable by-products. For that reason, the art dealing with acid hydrolysis of primarily cellulosic material is generally limited to the hydrolysis of waste paper and paper by-products or agricultural wastes such as corn hulls, husks or cobs. For example, U.S. Pat. No. 4,201,596 to Church et al. discloses a continuous process for the saccharification of cellulosic materials in a tubular reactor with an acid catalyst. The object of the Church et al. process is the conversion to glucose, furfural and xylose of cellulosic waste materials such as saw dust, wood waste, corncobs, etc. Along the same lines, the kinetics of the conversion of cellulosic wastes to monosaccharides in a plug flow reactor are described in Thompson, David R. and Grethlein, James E. "Design and Evaluation of a Plug Flow Reactor for Acid Hydrolysis of Cellulose." Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 3, pp 166-169 (1979). The authors of said article are specifically interested in hydrolyzing cellulose-rich material to monosaccharides. The authors do not disclose a method of hydrolyzing only to oligomers, much less to a specific mix of oligomers. Another disclosure, U.S. Pat. No. 4,316,747 to Rugg et al., describes a process for hydrolyzing cellulosic waste to glucose using an acid catalyst in a twin screw extruder.
Though the art discloses much about the short time, high temperature acid hydrolysis of cellulose-rich materials, the art does not disclose such treatment of materials in which cellulose is not a major component, such as a coffee extraction residue material, particularly the spent grounds from a commercial percolation system. The major hydrolyzable carbohydrate in coffee extraction residue material is mannan, not cellulose. Moreover, the products of mannan hydrolysis degrade under cellulose hydrolysis conditions, destroying any desirable mannan oligomer intermediates that are produced.
It is an object of the present invention to provide a method of hydrolyzing a coffee extraction residue material in which mannan is the major carbohydrate.
Another object of the invention is to provide a method of hydrolyzing a coffee extraction residue material to produce a mannan oligomer mixture having oligomers from DP 1 to about DP 10.
Still another object of the invention is to provide a method for producing a mannan oligomer mixture with the desired distribution of oligomers between DP 1 and about DP 10.